1. Field of the Invention
The present invention relates to a liquid crystal display and more particularly to a liquid crystal display and a fabricating method thereof capable of improving brightness and contrast.
2. Description of the Related Art
In general, a liquid crystal display LCD displays pictures by applying an electric field to the liquid crystal in response to a video signal to control an arrangement state of the liquid crystal, thereby adjusting a light transmittance in accordance with a video signal. The liquid crystal used in the liquid crystal display has a mid-state between solid and liquid thus having fluidity as well as elasticity. To date, the liquid crystal typically used in liquid crystal display is a twisted nematic mode (TN mode). Although the response speed of the twisted nematic mode may be different in accordance with the physical property of a particular type of liquid crystal material, such as cell gap, the response speed of the twisted nematic mode is longer than one 16.67 ms frame, as specified by National Television System Committee (NTSC) in a moving picture. Therefore, when the moving picture is displayed on the liquid crystal display of the twisted nematic mode, motion blurring, in which picture is seen ambiguously, and tailing seriously appears in the displayed picture.
In contrast with the twisted nematic mode above, the liquid crystal cell having a ferroelectric liquid crystal FLC injected therein readily implements moving pictures because its response speed is faster than that of the liquid crystal cells of any mode, including the twisted nematic mode. The ferroelectric liquid crystal inherently achieves permanent polarization, such as spontaneous polarization, without an external electric field. Therefore, when the external electric field is applied to the ferroelectric liquid crystal, the ferroelectric liquid crystal rotates rapidly because of the interaction between the external field and the spontaneous polarization. Thus, the response speed of ferroelectric liquid crystal is hundreds or thousands of times faster than that of the other liquid crystal modes.
The ferroelectric liquid crystal has a layer structure in which the electric and the magnetic properties are the same. The ferroelectric liquid crystal is driven by rotating individual crystals of the liquid crystal along a line of a virtual cone in response to the electric field. Since the ferroelectric liquid crystal has an in-plane-switching property in and of itself, a wide-viewing-angle can be implemented without having a special electrode structure or a compensation film.
Ferroelectric liquid crystal is classified into either a V-Switching Mode or a Half V-Switching Mode according to the characteristic response in reaction to the a polarity of the electric field. In the ferroelectric liquid crystal cell of V-Switching Mode, as the temperature is lowered, there occurs a thermodynamical phase transition, such as a sequence of an isotropic→a smectic A phase SA→a smectic X phase Sm X*→a crystal . Isotropic is the state in which liquid crystal molecules does not have direction or location order. The smectic A phase is a state in which the liquid crystal molecules are divided into a virtual layer and arranged vertically in the virtual layer and have an up and down symmetry. The smectic X phase is a middle state between the smectic A phase and the crystal phase. FIG. 1 is a graph illustrating a voltage versus transmittance characteristic for a ferroelectrics liquid crystal of V-Switching mode. The ferroelectric liquid crystal cell of V-Switching Mode in which the liquid crystal molecule is transited to the smectic X phase, as shown in FIG. 1, improves the light beam transmittance of incident light by changing the arrangement state responding to an external voltage of positive polarity +V and an external voltage of negative polarity −V.
The ferroelectric liquid crystal cell of the V-Switching Mode has the advantages of high-speed-response characteristic and wide-viewing-angle characteristic but has a defect in that a large amount of power is needed to drive a liquid crystal cell because the spontaneous polarization value is large. Further, the capacitance of a storage capacitor to maintain the data voltage has to be large. Accordingly, if the V-Switching Mode is used in a liquid crystal display, the aperture ratio becomes low since an electrode area of auxiliary capacitor becomes large. In contrast with the V-Switching Mode above, the ferroelectric liquid crystal cell of the Half V-Switching Mode has the advantages of a high-speed-response characteristic and a wide-viewing-angle characteristic. Further, a moving picture can be displayed with a high aperture ratio because capacitance is relatively small and thus requires less electrode area.
In respect to the Half V-Switching Mode shown in FIG. 2, a phase transition occurs from the isotropic phase to the nematic phase N* as the temperature is lowered below a transition temperature Tni. Below a transition temperature Tsn, a phase transition occurs from the nematic phase N* to the smectic C phase Sm C*. Below a transition temperature Tcs a phase transition from the smectic C phase to crystal. Thus, the thermodynamical phase transition occur in a sequence of isotropic→the nematic N*→the smectic C phase Sm C*→crystal.
Phase transitions will be further explained with reference to FIG. 3 in regard to the method for making a Half V-Switching Mode liquid crystal cell. FIG. 3 is a diagram illustrating a change of a molecule arrangement according to an electric field whether or not a ferroelectric liquid crystal is Half V-Switching mode. The ferroelectric liquid crystal is injected into cells having a parallel alignment films at the incipient temperature of the isotropic phase without direction or location order. If this temperature of ferroelectric liquid crystal in the isotropic phase is lowered to a designated temperature, the ferroelectric liquid crystal goes into the nematic phase N* arranged in parallel with respect to an alignment direction of the alignment films. In the nematic phase N*, if the temperature is gradually lowered and a sufficient alignment electric field is applied inside the liquid crystal cell, the ferroelectric liquid crystal of the nematic phase N* is transititions to the smectic phase C* and the spontaneous polarization direction of the ferroelectric liquid crystal is arranged coincident with the direction of electric field formed inside of cells. As a result, the ferroelectric liquid crystal within the liquid crystal cells has an entirely uniform alignment condition because the spontaneous polarization direction coincides with the direction of an alignment electric field in a two-molecule arrangement direction and the aligned direction of the alignment films disposed on both the upper and lower plates. On the other hand, without the alignment electric field process, the two-molecule arrangement appears as randomly different layers in a transition from the nematic phase N* to the smectic C phase Sm*C. If the bistable state in which the molecular arrangement of the ferroelectric liquid crystal appears random, uniform control of the ferroelectric liquid crystal becomes difficult. Therefore, the ferroelectric liquid crystal cell of the Half V-Mode should be arranged to be in a monostable state by transitioning the ferroelectric liquid crystal from the nematic phase N* to the smectic C phase Sm C* by applying a small DC voltage for an alignment electric field as temperature lowers. In FIG. 3, the “{circle around (X)}” represents the alignment electric field direction and the spontaneous polarization direction of the ferroelectric liquid crystal coinciding with the direction that vertically enters the drawing in FIG. 3 from above the paper or perpendicular to the plane shown in FIG. 3.
The liquid crystal display adapting the ferroelectric liquid crystal cell of the Half V-Switching Mode has electrodes on its upper and lower plates for vertically applying electric fields across the ferroelectric liquid crystal. Polarizers that are oriented ninety degrees with respect to each other are respectively positioned on the upper and the lower plates.
FIG. 4A and FIG. 4B are graphs illustrating the changes of light beam transmittance with respect to voltage changes for the ferroelectric liquid crystal cell of the Half V-Switching Mode. As shown in FIG. 4A, while being aligned under an alignment electric field from the voltage of a negative polarity −V or the electric field of a negative polarity, the ferroelectric liquid crystal cell of the Half V-Switching Mode allows incident light beam to transmit by converting the polarization direction of the incident light beam to 90° only when a positive voltage +V is applied, and allows incident light beam to cut-off nearly by maintaining the polarization direction of the incident light beam when a negative voltage −V is applied thereto. The light beam transmittance is increased in proportion to the intensity of the positive electric field and is maintained at a maximum value if the intensity of positive electric field increases to more than a designated threshold value.
On the contrary, when the ferroelectric liquid crystal of the Half V-Switching Mode cell is aligned under an alignment electric field from the voltage of a positive polarity +V or the electric field of positive polarity, the ferroelectric liquid crystal cell of the Half V-Switching Mode, as illustrated in FIG. 4B, permits incident light beam to transmit only when a negative voltage −V is applied thereto and nearly cuts off incident light beam when a positive voltage +V is applied thereto.
FIG. 5 is a diagram illustrating a ferroelectric liquid crystal of Half V-Switching mode reacting to an applied electric field when being driven by alignment electric field. More specifically, FIG. 5 represents the change of the arrangement of the ferroelectric liquid crystal under an alignment electric field of negative polarity and the arrangement of the ferroelectric liquid crystal changes when the external electric field of positive polarity and negative polarity is applied to the ferroelectric liquid crystal cell the Half V-Switching Mode. As shown in FIG. 5, when the ferroelectric liquid crystal cell of Half V-Switching Mode is aligned under an electric field by an external electric field of negative polarity, the spontaneous polarization direction Ps of the ferroelectric liquid crystal is uniformly aligned to the direction coinciding with the external electric field of negative polarity. After the external electric field of positive polarity E+ is applied to the ferroelectric liquid crystal cell, the arrangement of ferroelectric liquid crystal is changed and the spontaneous polarization direction Ps coincides with the external electric field of positive polarity. At this moment, the polarization direction of incident light beam from the lower plate of liquid crystal display is changed to the polarization direction of an upper polarizer by the ferroelectric liquid crystal that has a changed arrangement such that the incident light beam transmits through the upper plate. When the external electric field of negative polarity E− is applied or the external electric field is not applied to the ferroelectric liquid crystal cell of the Half V-Switching Mode, the arrangement of the ferroelectric liquid crystal still maintains the incipient arrangement state and the incident light beam is cut off by the ferroelectric liquid crystal cell because the polarization direction of the ferroelectric liquid crystal is maintained.
The ferroelectric liquid crystal cell of the Half V-Switching mode has a defect in that light leakage occurs since a long axis of the liquid crystal molecules is tilted somewhat in response to the arrangement direction of the alignment films. The upper and lower alignment films of the ferroelectric liquid crystal cell are aligned in opposite directions. As shown in FIGS. 6A and 6B, the ferroelectric liquid crystal molecules adjacent to the upper and lower plates are tilted to the right with respect to the alignment direction of the alignment film. In this regard, a pre-tilt angle θ of the ferroelectric liquid crystal molecule to the alignment direction becomes approximately 5 degrees. As shown in FIG. 6C, the pre-tilt angles appear to be in the same direction when the upper and lower plates are assembled in the same direction, since the upper plate is turned over and attached to the lower plate.
The light transmittance characteristic of the ferroelectric liquid crystal cell when no voltage is applied should be a black state such that no light is transmitted. However, light is transmitted due to the ferroelectric liquid crystal molecules adjacent to the alignment film surface having a pre-tilt such that the liquid crystal cell has a brightness more than that of the black state. For example, if the voltage of a negative polarity −V is applied at incipient alignment of the ferroelectric liquid crystal, the ferroelectric liquid crystal cell reacts upon the voltage of positive polarity +V and thus induces the change of the light transmittance in accordance with the change of the voltage of the positive polarity +V. As described above, if the ferroelectric liquid crystal molecules have a pre-tilt angle θ, a reaction does not arise, as described with respect to FIG. 4A, and the light is not cut off completely in respect to the voltage of the negative polarity and some light is transmitted, as shown in FIG. 7. In respect to the voltage of the positive polarity +V, a sufficient amount of light cannot be transmitted. So in the ferroelectric liquid crystal of the prior art, the leakage phenomenon of light also arises upon black state and the contrast ratio is low and the brightness is deteriorated and further implement of the gray scale embodiment becomes difficult.